Although only a limited number of studies have been conducted on aluminum compounds which themselves have radical generating ability, a possibility exists that such compounds might be used as radical generators. For instance, cases have been reported where mixing of dichloroethylaluminumacrylonitrile complex with butadiene resulted in radical copolymerization [W. Kuran et al., "Die Makromolekulare Chemie", 174, 73-80 (1973)] or radicals were detected in ESR spectrometry of oxidation products from alkyldiphenoxyaluminum compounds [Pasynkiewicz et al., "Journal of Organometallic Chemistry", 141, 149-156 (1977)]. Such reports suggest that aluminum compounds have radical generating ability.
While few studies have dealt with the radical generating ability of ate complexes of aluminum (aluminate complexes), it can be expected by analogy that the ate complexes might have radical generating ability. However, organoaluminum compounds are generally unstable in air and difficult to handle during their synthesis. For these and other reasons, few attempts have so far been made to use them as radical generators for various purposes.
On the other hand, photopolymerizable compositions basically contain a photopolymerization initiator and an addition-polymerizable compound having two or more ethylenically unsaturated bonds (hereinafter referred to as "polyfunctional monomers"). Upon irradiation with light, said compositions harden, show changes in tackiness and/or become insoluble in solvents. Owing to these properties, such compositions are widely used in photography, printing, metal surface treatment and inks, among others. Theories and application examples are described in a number of monographs. For example, detailed description can be found in J. Kosar: "Light Sensitive Systems", John Wiley & Sons, New York, 1965, pages 158-193; and K. I. Jacobson and R. E. Jacobson: "Imaging Systems", John Wiley & Sons, New York, 1976, pages 181-222.
Recently, image-forming systems which utilize light-sensitive microcapsules containing a photopolymerizable composition have been proposed as a method of forming images using a photopolymerizable composition. Thus, for instance, a method of forming color images which comprises exposing to light a color-forming sheet coated with microcapsules containing a dye plus a photopolymerizable composition composed of a vinyl compound and a photopolymerization initiator, then placing an image-receiving sheet over the exposed color-forming sheet and applying pressure on the whole set of sheets is disclosed in JP-A-57-124343, JP-A-57-179836 and JP-A-57-197538 (the term "JP-A" as used herein means an "unexamined published Japanese Patent Application").
Furthermore, in recent years, the possibility of applying such photopolymerizable compositions to full-color light-sensitive materials spectrally sensitized down to the visible light range has been investigated.
Thus, for example, a method of sensitizing a photopolymerizable composition is described in "Nippon Shashin Gakkaishi" (Journal of the Society of Photographic Science and Technology of Japan), 49 (3), 230 (1986) and "Kino Zairyo" (Function & Materials), September 1983 issue, pages 48-60. A method of spectrally sensitizing an organic peroxide initiator with organic dyes is described in JP-A-59-89340. Furthermore EP-A-223587 discloses organic cationic dye-organoboron anion salts as novel photopolymerization initiators.
In particular the method using organic cationic dye compound-organoboron compound anion salts as light-sensitive initiators is an advantageous method since the cationic dye compounds can be selected within a wide range and therefore photopolymerizable compositions sensitive to an arbitrarily selected wavelength can be designed.
However, boron compounds are generally expensive, while organoaluminum compounds are generally unstable in air. Consequently their commercial use has been limited.
Meanwhile, various image-forming materials are known. Thus, for example, silver halide photographic materials, materials in which a photopolymer is used (e.g. materials for lithography and relief printing), diazo photographic materials, and image-forming materials involving photobleaching of dyes are known.
Among these materials, the image-forming materials utilizing the dye photobleaching method which comprises photobleaching of dyes in imagewise exposed portions to give an image are characterized in that the image forming procedure is easy and simple and the process is a dry one and that in principle, the image-forming materials can readily modified for color imaging.
For photobleaching of dyes, methods using organo-boron compounds are known. Thus, for instance, combinations of organoboron compounds and bleachable dyes are described, for example, in British Patents Nos. 1,386,269, 1,370,058, 1,370,059 and 1,370,060 and U.S. Pat. No. 4,307,182. These organoboron compound-bleachable dye combinations can readily and directly form images by irradiation with light.
The light-sensitive materials in which an organoboron compound-bleachable dye combination is used have been variously modified. For instance, U.S. Pat. No. 4,307,182 discloses a method of fixing images with an acid, and European Patent 109,772 discloses a method of fixing images with ultraviolet radiation. Furthermore, U.S. Pat. No. 4,450,227 discloses a method of reducing the solvent quantity or attaining color imaging by dispersing such an organoboron compound-bleachable dye combination in a binder.
However, in spite of such various modifications, organoboron compounds are generally expensive, as mentioned above, limiting the commercial use of the materials mentioned above.